Lab: Modern Atmospheric CO2 Record

I. Introduction

The measurements of atmospheric carbon dioxide (CO2) concentrations
at the Mauna Loa Observatory are derived from the Scripps Institution of
Oceanography's continuous monitoring program. This record constitutes the
longest continuous record of atmospheric CO2 concentrations available
in the world. Monthly average mole fractions of CO2 in water-vapor-free
air are given from March 1958 through 1992, except for a few interruptions.
For comparison, CO2 records for two other stations, Barrow (Alaska),
and South Pole (Antarctica) are given. The comparison of the records yield
insights into natural and anthropogenic sources of CO2 and its
atmospheric transport.

A. Atmospheric CO2 Data

Open the atmospheric CO2 data. Familiarize
yourself with the location of the stations where the data were collected
by checking out the related web sites: Mauna
Loa, South
Pole, Barrow.
Note that there is one parameter, CO2, measured monthly at three
locations: Mauna Loa (Hawaii), South Pole (Antarctica), and Barrow (Alaska).

Task 1: What are the general characteristics of this data
set? Consider the units of CO2, and the maximum and minimum CO2 concentrations
for each station. Overall, are the values increasing or decreasing with time?

Task 2: What is the cause of the short-period oscillation
of CO2?
To answer this question, make two charts of the data: one showing all the
data and the other showing the last 3 years of the CO2 records.
Include data from all three stations. Recall that this is an "xy scatter
plot" and
not a "line
plot" when using Excel's chart function.

How long is each oscillation?

When do maxima and minima occur at the individual stations? Why?

What are the differences in amplitude and timing in the oscillations
in the three CO2 records?
Can you explain the differences?

Now review this animation showing
monthly variations in net primary productivity. Discuss these seasonal
changes in the context of the time series.

Task 3: Calculate the annual averages for each of the last
three years for the three sites and compare them. What causes the differences
in annual average CO2 concentrations at the sites?

Task 4: What is responsible
for the long-term change in CO2 concentrations
measured at Mauna Loa? Go back to the graph showing the entire time series.
Determine the long-term rate of change of the Mauna Loa CO2 concentrations
by adding a trendline and displaying
the equation of that line on the graph. What is the rate of change (including
units)? Using the regression equation, extrapolate CO2 concentrations
to the year 2100. How long will it take
for CO2 to rise by 50% of the last
measurement at this rate?

So far, you have assumed that the rate of
increase in CO2 is
constant. Is there evidence that the rate is actually systematically
increasing or decreasing over the years? Do you have an explanation
of this phenomenon? Based on the shape of the curves, do you think that you
over- or underestimated the time it will take to exceed the latest
measurement by 50%?

B. Greenhouse Gases

Task 5: Examine this table showing
the concentrations, global warming potentials, and lifetimes for various
greenhouse gases (GHGs). Answer the following questions:

Which GHGs are most
abundant? Note which are measured in ppm and which in ppb? In what units
are the others measured?

The Global Warming Potential (GWP) is typically
used to contrast different greenhouse gases relative to CO2.
GWP is an index for estimating relative global warming. It is a measure
of warming contribution of a kg of a particular
GHG as compared to the warming contibution of a kg of carbon dioxide.
The GWP provides a simple measure of the relative radiative effects of
the emissions of various greenhouse gases (see footnote 2 in the table
for more information). What are the Global Warming Potentials of CO2 and
CH4? Which 3 GHGs have the highest GWP? Which
3 have the lowest?

Atmospheric lifetime is the time-scale characterizing
the decay of an instantaneous pulse input into the reservoir (see footnote
3). What are the lifetimes of CO2 and CH4?
Which 3 GHGs have the longest lifetimes? Which 3 have the shortest?

Why do CO2 concentrations
increase for all scenarios, even though emissions have decreased
for some of them? What is the minimum CO2 concentration
projected for 2100? How does this compare with your projection
for 2100 based on extrapolation of the Mauna Loa trend(s)?

Why do CH4 concentrations show a decrease for some
scenarios? Do the decrease in CH4 concentration and CH4 emission
occur at the same time? Why or why not?

Task 8: HCFC-22 and HFC-23 were designated
acceptable replacements for CFCs-11/12/113 in the initial Montreal Protocol
because they have less potential to deplete stratospheric ozone than
the CFCs. However, HCFC-22 and HFC-23 are greenhouse gases.

Go
back to the Current
Greenhouse Gas chart. How do the present tropospheric concentrations,
GWP, and atmospheric lifetimes of HCFC-22 and HFC-23 compare to those
of the CFCs (11/12/113)?

What are the major chemical differences between
CFCs, HCFCs and HFCs? (see Kump Chapter 17 especially pages 357-359)
Why are HCFCs and HFCs used as substitutes for CFCs? What are the environmental
consequences of using HCFC-22 and/or HFC-23?

III. Data

VI. Lab Report Instructions

Write a lab report (as per the Lab Report Format) summarizing the major findings
of your investigation including the answers to the questions given in the
lab instructions. Include the following questions in your discussion.

What processes control atmospheric CO2 concentrations on an annual basis
at different latitudes?